Controlled Interfacial Adsorption of AuNW Along 1-Nm Wide Dipole Arrays on Layered Materials and The Catalysis of Sulfide Oxygenation
Controlling the surface chemistry of 2D materials is critical for the development of next generation applications including nanoelectronics and organic photovoltaics (OPVs). Further, next generation nanoelectronics devices require very specific 2D patterns of conductors and insulators with prescribed connectivity and repeating patterns less than 10 nm. However, both top-down and bottom-up approaches currently used lack the ability to pattern materials with sub 10-nm precision over large scales. Nevertheless, a class of monolayer chemistry offers a way to solve this problem through controlled long-range ordering with superior sub-10 nm patterning resolution. Graphene is most often functionalized noncovalently, which preserves most of its intrinsic properties (i.e., electronic conductivity) and allows spatial modulation of the surface. Phospholipids such as 1,2-bis(10,12-tricsadiynoyl)-sn-glycero-3-phosphoethanolamine (diyne PE) form lying down lamellar phases on graphene where both the hydrophilic head and hydrophobic tail are exposed to the interface and resemble a repeating cross section of the cell membrane. Phospholipid is made up of a complex headgroup structure and strong headgroup dipole which allows for a diverse range of chemistry and docking of objects to occur at the nonpolar membrane, these principals are equally as important at the nonpolar interface of 2D materials. A key component in the development of nanoelectronics is the integration of inorganic nanocrystals such as nanowires into materials at the wafer scale. Nanocrystals can be integrated into materials through templated growth on to surface of interest as well as through assembly processes (i.e. interfacial adsorption).
In this work, I have demonstrated that gold nanowires (AuNWs) can be templated on striped phospholipid monolayers, which have an orientable headgroup dipoles that can order and straighten flexible 2-nm diameter AuNWs with wire lengths of ~1 µm. While AuNWs in solution experience bundling effects due to depletion attraction interactions, wires adsorb to the surface in a well separated fashion with wire-wire distances (e.g. 14 or 21 nm) matching multiples of the PE template pitch. This suggests repulsive interactions between wires upon interaction with dipole arrays on the surface. Although the reaction and templating of AuNWs is completed in a nonpolar environment (cyclohexane), the ordering of wires varies based on the hydration of the PE template in the presence of excess oleylamine, which forms hemicylindrical micelles around the hydrated headgroups protecting the polar environment. Results suggest that PE template experience membrane-mimetic dipole orientation behaviors, which in turn influences the orientation and ordering of objects in a nonpolar environment.
Another promising material for bottom-up device applications is MoS2 substrates due to their useful electronic properties. However, being able to control the surface chemistry of different materials, like MoS2, is relatively understudied, resulting in very limited examples of MoS2 substrates used in bottom-up approaches for nanoelectronics devices. Diyne PE templates adsorb on to MoS2 in an edge-on conformation in which the alkyl tails stack on top of each other increasing the overall stability of the monolayer. A decrease in lateral spacing results in high local concentrations of orientable headgroups dipoles along with stacked tails which could affect the interactions and adsorption of inorganic materials (i.e. AuNW) at the interface.
Here, I show that both diyne PE/HOPG and diyne PE/MoS2 substrates can template AuNW of various lengths with long range ordering over areas up to 100 µm2. Wires on both substrates experience repulsive interactions upon contact with the headgroup dipole arrays resulting in wire-wire distances greater than the template pitch (7 nm). As the wire length is shortened the measured distance between wires become smaller eventually resulting in tight packed ribbon phases. Wires within these ribbon phases have wire-wire distances equal to the template. Ribbon phases occur on diyne PE/HOPG substrates when the wire length is ~50 nm, whereas wire below ~600 nm produce ribbon phases on diyne PE/MoS2 substrates.
Another important aspect to future scientific development is the catalysis of organic reactions, specifically oxygenation of organic sulfides. Sulfide oxygenation is important for applications such as medicinal chemistry, petroleum desulfurization, and nerve agent detoxification. Both reaction rates and the use of inexpensive oxidants and catalysts are important for practical applications. Hydrogen peroxide and tert-butyl hydroperoxide are ideal oxidants due to being cost efficient and environmentally friendly. Hydrogen peroxide can be activated through transition metal base homogeneous catalysts. Some of the most common catalysts are homo- and hetero-polyoxometalates (POMs) due their chemical robustness. Heptamolybdate [Mo7O24]6- is a member of the isopolymolybdate family and its ammonium salt is commercially available and low in cost.22 Heteropolyoxometalates have been widely studied as a catalyst for oxygenation reactions whereas heptamolybdate has been rarely studied in oxygenation reactions.
Here I report sulfide oxygenation activity of both heptamolybdate and its peroxo derivate [Mo7O22(O2)2]6-. Sulfide oxygenation of methyl phenyl sulfide (MPS) by H2O2 to sulfoxide and sulfone occurs rapidly with 100 % utility of H2O2 in the presence of [Mo7O22(O2)2]6-, suggesting the peroxo adduct is an efficient catalyst. However, heptamolybdate is a faster catalyst compared to [Mo7O22(O2)2]6- for MPS oxygenation and all other sulfides tested under identical conditions. Pseudo-first order kcat constants from initial rate kinetics show that [Mo7O24]6- catalyzes sulfide oxygenation faster. The significant difference in the kcat suggests differences in the active catalytic species, which was characterized by both UV-Vis and electrospray ionization mass spectrometry. ESI-MS suggest that the active intermediate of [Mo7O24]6- under catalytic reaction conditions for sulfide oxygenation by H2O2 is [Mo2O11]2-. These results show that heptamolybdate is a highly efficient catalyst for H2O2 oxygenation of organic sulfides.